High efficiency radiant heater

ABSTRACT

A radiant tube heater with a burner assembly, a radiant tube assembly and a combustion air pre-heater, wherein the burner assembly comprises: a burner fuel nozzle; a plenum chamber and a pre-mixer chamber; the plenum chamber having a combustion air inlet; wherein in use: combustion air flows from said plenum chamber through an orifice to said pre-mixer where said air, is mixed with burner fuel entering said pre-mixer through said nozzle prior to being combusted at a burner head; said pre-mixer being at least partly located within said radiant tube assembly; and where in use at least part of the combustion air supplied to said plenum is preheated in said air pre-heater using residual sensible heat of the hot combustion gas products of the heater.

BACKGROUND OF INVENTION

1. Field of the Invention

This invention relates to a gas fired radiant tube heater with a highradiant heat efficiency.

2. Description of the Related Art

Gas fired radiant tube heaters are widely used especially to heatindustrial and commercial premises. When so used it is desirable thatsuch heaters have a high radiant heat efficiency; that is that a highproportion of the potential energy of the fuel gas is converted toradiant heat, rather than, for example, convective heat. This makes theheater more suitable for location in an elevated position allowinguseful heat to be radiated downwardly (often toward occupants of thepremises). However, it has generally been found that high radiantefficiency heaters can only be obtained at the expense of a reducedoverall thermal efficiency.

BRIEF SUMMARY OF INVENTION

It is an object of the present invention to provide a radiant tubeheater having a higher than normal radiant efficiency while at the sametime having a good overall thermal efficiency. It is also an aim of thepresent invention to provide a more compact heater.

In one aspect the invention comprises a radiant tube heater with aburner assembly, a radiant tube assembly and a combustion airpre-heater; wherein the burner assembly comprises: a burner fuel nozzle;a plenum chamber and a pre-mixer chamber; the plenum chamber having acombustion air inlet; wherein in use: combustion air flows from saidplenum chamber through an orifice to said pre-mixer where said air ismixed with burner fuel entering said pre-mixer through said nozzle priorto being combusted at a burner head; said pre-mixer being at leastpartly located within said radiant tube assembly; and where in use atleast part of the combustion air supplied to said plenum is preheated insaid air pre-heater using residual sensible heat of the hot combustiongas products of the heater.

Preferably, the burner fuel nozzle comprises an axial outlet and aplurality of radial outlets. This advantageously helps promote goodmixing of air and fuel in the pre-mixer.

Preferably the radiant tube heater further comprises a secondary airchannel that surrounds said pre-mixer and is in fluid communication withsaid plenum chamber such that in use air from said plenum enters saidsecondary air channel and thereby supplies secondary air for furthercombustion of products leaving said burner head. Preferably, thissecondary air channel is located within the entry portion of the radianttube assembly. This advantageously allows a relatively long pre-mixertube to be used while maintaining a compact sized heater and alsosimplifies the design of the plenum chamber. Preferably the burner headextends over the exit portion of said secondary air channel.

The air pre-heater may comprise one or more cross-flow type heatexchanger blocks. Preferably, in use, combustion air is preheated bypassing in a first direction through at least part of a heat exchangerblock and in a second direction through a further heat exchanger. Morepreferably, in use during preheating the combustion air makes a U-turnsuch that said first direction is in an opposite direction to saidsecond direction, or the combustion air makes a 90° turn. Thisadvantageously allows the heat exchanger unit to be located close to theburner assembly. During preheating the combustion air may make a U-turnsuch that said first direction is in an opposite direction to saidsecond direction. Changing gas flow direction in the above wayadvantageously allows a more compact radiant tube heater.

Preferably, the first heat exchanger block and the second heat exchangerblock are, at least in part, side-by-side. This advantageously allows amore compact radiant tube heater. Preferably, the radiant tube assemblyis U-shaped and preferably comprises two straight tubular portionsconnected by a semi circular tubular portion. This provides a morecompact heater unit.

The combustion gas leaving the radiant tube assembly may enter the airpre-heater in a first direction and may leave the air pre-heater in asecond direction, wherein said second direction is generally orthogonalto said first direction.

Preferably, the radiant tube assembly has a reflector unit andinsulation means is provided on at least part of said reflector unit.The insulation means may comprise an insulating panel wherein thereflector is located between said insulating panel and said radiant tubeassembly such that an air gap separates at least a major part of saidreflector unit from said insulating panel. Alternatively, the insulationmeans may comprise an insulating panel and wherein the reflector islocated between said insulating panel and said radiant tube assemblysuch that a layer of refractory insulation separates at least a majorpart of said reflector unit from said insulating panel. Thisadvantageously increases the efficiency of the radiant tube assembly inradiating heat energy.

The radiant tube assembly may have a reflector unit and an over-shieldwherein the over-shield is located between said reflector and saidradiant tube assembly. This advantageously increases the efficiency ofthe radiant tube assembly in radiating heat energy

In another aspect the invention comprises a radiant tube heater having aburner assembly, a radiant tube assembly and a heat exchanger assembly,said heat exchanger assembly comprising two or more cross-flow type heatexchanger blocks; wherein in use residual sensible heat in thecombustion products of said burner assembly is used to preheatcombustion air supplied to said burner assembly, by passing saidcombustion air in a first direction through a first heat exchanger blockand then in a second direction through a second heat exchanger block.This allows the heater to be more compact.

BRIEF DESCRIPTION OF DRAWINGS

A preferred embodiment of the invention will now be described byreference to the following diagrammatic figures in which:

FIG. 1 shows an overall (partly exploded) perspective view of a heateraccording to preferred embodiment of the invention;

FIG. 2 shows a side elevation of burner assembly of the heater of FIG.1;

FIG. 3 shows a side cross section of the radiant tubes and the reflectorassembly of the heater of FIG. 1;

FIG. 4 shows a perspective view of the heat exchanger assembly of theheater of FIG. 1;

FIG. 5 shows a perspective view of FIG. 4 in exploded form;

FIG. 6 shows the heat exchanger block of the heat exchanger at assemblyof FIG. 4; and

FIG. 7 shows a second embodiment of the invention with an over-shield.

DETAILED DESCRIPTION OF THE INVENTION

A preferred embodiment of the invention will now be described byreference to FIGS. 1-6.

FIG. 1 shows a radiant tube heater 10 comprising; a fuel burner assembly12; a combustion air fan 14; a radiant tube assembly 16; areflector/insulator unit 18; and an air pre-heater heat exchanger 20. Inuse, fuel (typically natural gas) is combusted with preheated air(slightly compressed by passing through fan 14) in burner assembly 12.Hot combustion gases from the burner 12 pass into and through theU-shaped radiant tube assembly 16 before entering heat exchanger 20where residual sensible heat in the hot combustion products is used topreheat combustion air fed to burner assembly 12 (via fan 14); typicallyto a temperature of between 100 and 120° C. Above and around the sidesof the radiant tube assembly 16 there is located a reflector unit. Thepurpose of the reflector unit is to reflect radiant heat energydownwardly (that is away from the reflector) such that in use themaximum amount of radiant energy is directed in a downward directiontowards the floor of the space being heated.

FIG. 2 shows a side elevation (cross section) of the burner assembly 12and also the entry portion of the radiant tube assembly 16. Gaseous fuel(G) enters a control valve 22 where the fuel flow-rate is regulated andpasses from the exit of valve 22 into a straight nozzle feed pipe 24 andthence to burner nozzle 26. Burner nozzle 26 comprises at least oneaxial outlet orifice 28 and a plurality of radially directing orifices30. The burner nozzle 26 ejects fuel into a pre-mixer 32 where the fuelis premixed with combustion air (A). The axial and radial orifices 28and 30 help promote turbulence in the pre-mixer and generally promotegood mixing of air and fuel prior to combustion. This advantageouslyencourages a long flame and results in a more consistent flame quality.Combustion air enters the pre-mixer 32 through one or more orifices 34.Air feed to the burner is drawn through combustion air pre-heater 20 (asa result of suction fan 14) where it is preheated prior to enteringplenum chamber 36 located between control valve 22 and burner nozzle 26and surrounding feed pipe 24. Thus, gaseous fuel flowing through pipe 24is preheated owing to the higher temperature of the pre-heated airflowing through plenum chamber 36. Primary air orifices 34 are locatedin a wall that otherwise separates the plenum chamber 36 and thepre-mixer 32. Pre-mixer 32 comprises a cylindrical tube 38 and ispreferably formed by extruding aluminium or an alloy thereof. Pre-mixer32 is located within the radiant tube assembly 16 such that an annularchannel 42 is formed along which pre-heated air from plenum chamber 36may flow via one or more secondary air orifices 44. This simplifies thedesign of the plenum chamber 36. The downstream end of the pre-mixertube 38 terminates at a burner head 46 that may comprise an array ofcircular or rectangular orifices. Burner head 46 extends radiallyoutwardly beyond pre-mixer 32 so that an annular portion, containing aring of circumferentially spaced apertures 50, partly seals thedownstream end of annular channel 42. Burner head 46 ensures that thebase of flame (F) is retained on the exit surface of burner head 46.Pre-heated secondary air enters the flame F by flowing along annularchannel 42 and through apertures 50. This advantageously, assists mixingof secondary air with the products of combustion the secondary airenters flame F. Ignition means 48 (for example spark ignition) isprovided to allow initial lighting of the burner.

FIG. 3 shows further details of the reflector unit 18, located above thetubes 52 of the radiant tube assembly 16. The reflector unit is locatedsome way above the radiant heater tubes and extends part away around theouter side of each tube, the order to maximise the extent of downwardreflection of radiant heat energy. The invention provides insulationmeans 54, located at least in part proximal to reflector unit 18. Thisinsulation means 54 may comprise a layer of refractory material or thelike or alternatively may be provided by other insulating material suchas a substantially enclosed air pocket. The latter may be provided byplacing an insulating panel 56 above and close to reflector unit 18.This is most effective if the layer of enclosed air (air pocket) isrelatively thin.

FIGS. 4-6 shows details of a heat exchanger unit 20. Referring to FIG. 5the combustion air pre-heater exchanger 20 comprises a heat exchangerblock 60 which is shown in more detail in FIG. 6. Heat exchanger block60 is formed from an array of heat exchanger ducts 62 arrangedside-by-side and providing an internal flow channel 64 through whichcombustion air flows during preheating. The array of heat exchangerducts 62 are held in place by end plates members 66. In use, hotcombustion products pass (preferably in a direction orthogonal topreheat air flow) through the channels 68 located between individualexchanger ducts 62. Assembly member 70 provides a space for thepreheated air to turn after passing through a first array of ducts (62a) before flowing in an opposite direction through a second array ofducts (62 b). Assembly member 70 is sealed to the lower panel 72 (seeFIG. 5) of the heat exchanger unit housing by means of a gasket 74. Aseparation assembly 76 similar to assembly member 70 but having anadditional central member 78 in combination with gasket 80 provides aseal with the upper portion 84 (see FIG. 5) of the heat exchanger unitand ensures that air entering the exchanger through inlet flange 86 mustflow through the first array of tubes 62 a and cannot bypass theexchanger.

The heat exchanger block is located within housing 90 which takes theform of a box having a hot combustion products inlet duct 92, a cooledcombustion products outlet duct 94, a combustion air inlet duct 86, anda preheated air outlet duct 88.

FIG. 5 shows the position of combustion air inlet duct 86 and preheatedair outlet duct 88 relative to the heat exchanger block 60. In thisexample, the heat exchanger block 60 comprises (see FIG. 6) 10 ducts 62that in use function as two separate blocks (62 a and 62 b) of 5 ducts.Thus, when assembled heat air pre-heater exchanger unit 20 directscombustion air through inlet duct 86 which then passes through internalflow channels of the first 5 ducts 62 a. The partly preheated air thenmakes a U-turn and passes (in the opposite direction) through theremaining 5 ducts 62 b and thence to preheated air outlet duct 88 andthence through a duct 94 to the inlet of air fan 14 (see FIG. 1).Further, in use combustion products pass horizontally through spaces of68 (located between individual exchanger ducts 62) thereby providing theenergy required to preheat the combustion air. In consequence, duringpassage of combustion products in said horizontal direction sensibleheat is lost resulting in further cooling of the combustion products.Thus, heat exchanger block 60 in combination with the heat exchangerhousing 90 provide a cross flow heat exchanger in which combustion airis partly preheated in a first portion of the exchanger block 62 a, thenmakes a U-turn and is further preheated as he passes in an oppositedirection through a second portion of the exchanger block 62 b.

Naturally, the exchanger block 60 can comprise a wide range ofindividual exchanger ducts 62 and is not limited to a 5 plus 5 ductarrangement. Indeed, it is possible to have exchanger blocks 60 were thefirst portion of the exchanger block has more or less ducts and thesecond portion of the exchanger block. Preferably, the first portionwill have either a slightly more ducts or an equal number of ducts thanthe second portion.

FIG. 7 shows a further embodiment of the invention similar to theembodiment described above in most respects apart from the reflector118, which in this embodiment is not insulated. Also. located betweenthe reflector and the radiant tube assembly 140 there are over-shields100. Over-shields 100 are preferably located slightly above each tube ofa U-shaped radiant tube assembly 140. The over-shields reduce the amountof radiant energy that is incident upon the reflector 118 located above.This results in an improved (useful) radiant energy efficiency as energylosses associated with heating the reflector are reduced (this energyultimately being dissipated largely as convective heat loss).

We claim:
 1. A radiant tube heater comprising a burner assembly, aradiant tube assembly and a combustion air pre-heater; wherein in use atleast part of the combustion air supplied to the heater is preheated insaid air pre-heater using the residual sensible heat of hot combustiongas produced by the heater; and wherein the air pre-heater comprises: aheat exchanger block having a first and second array of ducts arrangedside by side, such that each array is contacted with hot combustion gasproducts at the same temperature, and held in place by end platemembers; said heat exchanger block being arranged so that combustion airis preheated by passing inside and through the first array of ductsbefore turning and being further preheated by flowing in an oppositedirection inside and through the second array of ducts; and where hotcombustion gas products are cooled by passing in cross flow through asingle enclosure, located between said end plate members.
 2. A radianttube heater assembly according to claim 1 said heat exchanger blockbeing arranged so that combustion air to be preheated passes in a firstdirection inside and through the first array of ducts before turningthrough an angle of 90°; during which the air flows in a plane ordirection orthogonal to the direction of flow of said hot combustion gasproducts and then turning a further 90°; to flow in a second directioninside and through the second array of ducts; and where said seconddirection is opposite to said first direction.
 3. A radiant tube heaterassembly according to claim 2 the radiant tube heater furthercomprising: a single reflector unit located above both said straighttubes; and a panel of similar shape and size to said reflector unitlocated immediately above and close to said reflector unit such that incombination with said reflector unit it substantially encloses aninsulating air pocket; said air pocket being characterized by therelatively thin layer of air located between said reflector and saidpanel.